The science magazine of Imperial College

Universal alchemy

A ground breaking astronomical observation has been made of two neutron stars colliding, in what has been called a “kilonova”. The observation, made during mid-August and publicly released 16 October 2017, was the first of its kind, and answered several long-standing questions about the nature of our universe.

The collision occurred in a galaxy known as NGC 4993, around 130 million light years away. This was both the first time a neutron star collision was detected, and the first time gravitational waves and electromagnetic radiation were both simultaneously detected from the same source.

Neutron stars are incredibly dense, with more mass than our Sun and just a few miles across – only one step away from being a black hole. Gravitational waves are wobbles in space-time caused by cataclysmic cosmic events, and were only detected for the first time in 2015 at the LIGO detector in the US, which earned three physicists the 2017 Nobel Prize.

LIGO was also involved in this recent detection, in partnership with the Virgo detector in Italy. The outbreak of electromagnetic radiation, known as a gamma ray burst (GRB), was detected only 1.7 seconds afterwards by NASA’s Fermi Gamma-Ray Burst Monitor. Over seventy teams of researchers worldwide participated in the flurry of observation and analysis in the following weeks with the resulting paper having around 4000 authors.

The observation has answered vital questions. The collision produced heavy elements such as gold, whose source had previously only been theorised – nuclear fusion in stars can generate elements as heavy as iron, but no more. Up to half of the elements whose cosmic source was previously unknown have now been confirmed to be produced by neutron star collisions.

Furthermore, the GRB from the collision was short – less than 2 seconds. Although the afterglow of short GRBs had been observed before, this was the first time they had been observed directly from the event; their source is now confirmed to be colliding neutron stars.

The discovery is being heralded as the beginning of a new era of astrophysics, dubbed “multi-messenger astronomy”, which utilises the observation of multiple types of signal – such as both electromagnetic radiation and gravitational waves – from a single event to enhance our understanding of what occurs.

Even the aftermath of the collision is exciting; it constitutes either the smallest black hole or largest neutron star ever witnessed. Observation of the remains continues.